This invention relates to the field of fiber optics, more particularly to the field of pumping optical fibers to generate or increase output power from fiber optic based devices, and specifically to mechanisms for launching pumping light into optical fibers.
In the field of fiber optic systems, fiber optic guides transmit light power from a light source to a utilization device. Referring to FIG. 1, light source 10 transmits light signal PS 11 at wavelength xcexS through fiber 12 to utilization device 14. Couplings between light source 10, utilization device 14 and fiber 12 are well known in the art and are not shown. Fiber 12 includes core 16, cladding 18 and protective covering 20. Light source 10 typically provides the optical signals carrying information which propagates in the core. This fiber is considered a single-clad fiber. There are also double-clad fibers. A double-clad fiber has a core, a first cladding, a second cladding and the protective coating. In the double-clad case, while a single-mode signal can propagate in the core, a multi-mode signal can be coupled into the inner cladding, whereupon the inner cladding acts as a core for the second cladding.
Numerous applications require the generation or amplification of optical signals. Fiber optics systems used in a large variety of commercial and military applications, such as in telecommunications, inter-satellite optical communications, and for missile radar tracking systems, require generation and amplification of optical signals.
Fiber optic guides (xe2x80x9cfibersxe2x80x9d) typically have at least two essential parts. One part is the core where light propagates. The other part is cladding surrounding the core which creates conditions whereby the light propagates only in the core. These fibers are capable of transmitting single mode optical signals in the core without amplification, and produce a small amount of background loss. These can be considered xe2x80x9cregularxe2x80x9d fibers.
xe2x80x9cSpecialxe2x80x9d fibers providing a gain medium typically include a core doped with rare earth atoms such as erbium (Er), ytterbium (Yb), erbium-ytterbium (ErYb), neodymium (Nd), thulium (Tm), etc., and are utilized in applications requiring the generation or amplification of optical signals. When subjected to optical energy (typically 800-1400 nm wavelength depending on the gain medium), these special fibers have atoms excited to their upper lasing level, and when thus excited they are capable of generating or amplifying optical signals. The special fibers providing the gain medium may be spliced to regular fibers, which then transmit the optical signals which have been generated or amplified in the gain medium.
A typical fiber amplifier has a source of optical signal coupled to a rare earth doped xe2x80x9cspecialxe2x80x9d fiber gain medium. Coupled also to the gain medium is an optical xe2x80x9cpumpxe2x80x9d source to input optical power into the gain medium, and a utilization device to receive an amplified optical signal as output from the gain medium. Referring to FIG. 2, in a typical fiber optic amplification system gain medium 22 is coupled with source fiber 10 to permit light signal PS 11 at wavelength xcexS to be amplified when combined with pump, light signal PP at wavelength xcexP to provide amplified signal APS at wavelength xcexS for use by utilization device 14.
Those skilled in the art can appreciate that the more pump power that is coupled into a rare earth doped fiber, the more optical signal output is provided by the gain medium. One form of gain medium 22 is described in PCT Publication WO 96/20519, entitled xe2x80x9cA Coupling Arrangement Between A Multimode Light Source and An Optical Fiber Through An Intermediate Optical Fiber Lengthxe2x80x9d, wherein a progressively tapered fiber portion is fused to the inner cladding of a double clad fiber carrying an optical information signal in its core. This fused system is shown schematically in FIG. 3 of the present application. However, while the spliced coupling allows the ability to have multiple locations available to input the pump power into a single fiber and achieve power scalability with unrestricted access to both fiber ends, such fused fiber couplers are somewhat difficult to manufacture.
There are various ways to couple pump power into special fiber. In most applications, fiber lasers and amplifiers are end-pumped by single-mode diode lasers whose output is coupled directly into the core of the fiber. The maximum output power achieved with such pumping schemes is currently about 100 mW. This is partly because 100-200 mW is typically the maximum power level that can be coupled into a fiber core at the lowest transverse mode from a readily manufactured semiconductor laser.
Unfortunately, there are applications, such as for space communications, which require multi-watt levels of pumping. Such higher output powers are generally achieved by using double-cladding fibers. These fibers have a doped single-mode core surrounded by a multi-mode inner cladding that guides pump radiation along the fibers. Typically, the pump radiation is launched into the inner cladding at one of the fiber ends with some kind of coupling optics. The maximum output power of such devices is limited by the brightness of available pump diodes, but tens of watts of output power have been demonstrated at specific wavelengths. The drawbacks of such configurations lie in stringent high-brightness requirements for the pump sources, limited accessibility of fiber ends, and in the difficulties in scaling to higher powers.
Efficient optical pumping of a single-mode fiber laser or an amplifier presents a serious challenge, especially when high output powers are required. Typical end pumping requires high-brightness-pump sources, limits scalability to higher powers, and restricts access to fiber ends, and known side-pumping techniques are difficult to manufacture. Accordingly, there exists a need for an effective, easy to manufacture method and apparatus for use in pumping fiber lasers and amplifiers which provides access to both fiber ends, enables scalability to high output powers, and is relatively straightforward and inexpensive to manufacture.
The present invention addresses the above-identified needs, providing a method and apparatus to achieve efficient and scalable optical power pumping into a single fiber while allowing unrestricted access to both fiber ends. Moreover, devices embodying the invention may be made for use with either single- or double-clad fibers, and are relatively inexpensive to manufacture.
In accordance with the present invention, a pumping fiber section includes a doped optical fiber core surrounded by a cladding layer having a cladding index of refraction. The pumping fiber section has a plurality of launch sections defined along the fiber and separated from each other by absorptive sections. Each launch section is stripped down to a first inner cladding layer. The pumping fiber is arranged to bring the plurality of launch sections into close proximity to each other on a substrate. The substrate provides a transparent low refraction index material in optical contact with the cladding and imparts a common radius to the adjacent fiber sections to form an upwardly convex launch region. This launch region is then mated to a launch port which matches the cladding index of refraction. Mating is effected by polishing the launch region flat and gluing the launch port thereto with a low-loss, index matched optical glue. The launch port accepts optical pump power on one or more sides from a pump light source such as a laser diode stripe, and conveys the pump light into the plurality of launch sections.
In accordance with some preferred embodiments of the present invention the launch port guides light into the fiber coil from two directions. In one preferred embodiment, two diode stripes and two optical connectors direct light into two sides of the launch port, which is generally trapezoidal in section. In another preferred embodiment, a single diode stripe provides light into a single side of the launch port, but residual light re-emerging from the fiber into the launch port is reflected back into the fiber using suitable reflection within the launch port. In another preferred embodiment, one side of the launch port is transmissive at a first wavelength and reflective at a second, while the other side is conversely transmissive at the second wavelength and reflective at the first wavelength, so that pump light may be guided into the launch port from two directions while residual light is reflected back into the fiber to enhance efficiency. In yet another preferred embodiment, pump light from two laser diode stripes is first polarized transversely to each other, and is then combined in a beam-splitting device so that the input optical power is nearly doubled. The optical power thus obtained is delivered to one entry face of the launch port. In another preferred embodiment, optical power from a similar arrangement using two additional laser diode stripes may be delivered to the opposite side of the same launch port. In yet another preferred embodiment, two such combined sources providing light at two different wavelengths may be directed into two faces of a launch port, and each launch port face may be conductive at one of the wavelengths and transmissive at the other to form a high-power and efficient pumping mechanism. The present invention can use either single-clad or double-clad fibers.